The present invention relates to technology for data backup.
Storage systems can provide storage areas to host computers in units called virtual volumes. In general, the storage capacity of a virtual volume is fixed; but there exists technology to automatically expand the storage capacity of a virtual volume, by allocating to the virtual volume a partial area of a physical resource (hereafter called a “segment”), in response to an access request from a host computer, in order to increase the efficiency of storage area utilization. For example, a disclosure of Japanese Patent Laid-open No. 2003-15915 is related to such technology.
A virtual volume is a virtual logical volume. A storage system can show to a host computer (specifically, for example, to an application executed by the host computer) a virtual storage capacity (hereafter called a “virtual capacity”) different from the total storage capacity of one or more segments actually allocated to the virtual volume (hereafter called the “actual capacity”). By allocating a segment to a virtual volume, the actual capacity of the virtual volume increases by (is expanded by) the storage capacity of the segment. Consequently, a virtual volume can also be called a “capacity expansion volume”. Or, because a virtual volume is a logical volume which can be provided using “thin provisioning” technology, a virtual volume can also be called a “thin provisioning volume”.
For example, a method exists in which a host computer is connected to disk storage and a tape library apparatus by a communication network called a Storage Area Network (SAN), and data stored in the disk storage is backed up in volume units to the tape library apparatus by a server for backup processing (hereafter called a “backup server”). In this case, the backup server reads data from logical volumes of the disk storage, and stores the read-out data to the tape library apparatus.
When a logical volume for backup is a virtual volume as described above, the virtual volume appears to the backup server to be a virtual volume with a virtual capacity amount different from the actual capacity, and so at the time of backup, reading and writing of the virtual capacity amount is performed. In general, the actual capacity is smaller than the virtual capacity. For this reason, reading and writing of the virtual capacity amount includes unnecessary reading and writing.
Further, when for example backed-up data is restored, in a process opposite that of the backup process, and more specifically, when data is restored to a logical volume within the disk storage from the tape library apparatus via the backup server, writing occurs to the virtual volume which is the restore destination in the virtual capacity amount, and so a larger actual capacity is required than at the time of backup.
The above problems can similarly occur when the backup source is a storage device of a type different from the disk storage, and/or when the backup destination is a storage device of a type different from the tape library apparatus.